struct ChangeInfo {
  bytes script;
  bytes satoshis;
}

library TxUtil {
  static const bytes ZEROSAT = b'0000000000000000';
  static function mergePrevouts(bytes[6] prevouts) : bytes {
    bytes result = b'';
    loop (6) : index {
        bytes prevout = prevouts[index];
        result += prevout;
      }
    return result;
  }
  static function mergeSpentScripts(bytes[6] spentScriptsCtx) : bytes {
    bytes result = b'';
    loop (6) : index {
        bytes spentScript = spentScriptsCtx[index];
        result += pack(len(spentScript)) + spentScript;
      }
    return result;
  }
  static function buildOutput(bytes script, bytes satoshis) : bytes {
    int nlen = len(script);
    require(nlen <= 34);
    return satoshis + pack(nlen) + script;
  }
  static function checkIndex(int indexVal, bytes index) : bool {
    bytes indexByte = pack(indexVal);
    if(indexByte == b'') {
        indexByte = b'00';
      }
    return indexByte + b'000000' == index;
  }
  static function buildOpReturnRoot(bytes script) : bytes {
    return (b'0000000000000000' + pack(len(script)) + script);
  }
  static function getStateScript(bytes hashRoot) : bytes {
    return b'6a1863617401' + hashRoot;
  }
  static function getChangeOutput(ChangeInfo changeInfo) : bytes {
    return changeInfo.satoshis != TxUtil.ZEROSAT ? TxUtil.buildOutput(changeInfo.script, changeInfo.satoshis) : b'';
  }
}